1. Field of the Invention
The present invention relates to a method for making a fluid bed furnace having an external circulating system for use in various facilities used for purposes such as incinerating or burning, drying or vaporization, or heat decomposition.
2. Prior Art
A fluid bed furnace having an external circulation system (called as "a fluid bed type furnace" hereafter) comprises a riser or a heating vessel of a cylindrical shape in which a fluid bed is formed by installing solid particles therein as heat transporting material, for heating material thereby for the purpose of burning, drying, vaporizing or decompositing the material. The heat transporting material are drawn out from an outlet pipe equipped at the top of the riser, and sent to a cyclone separator equipped at the end of the outlet pipe to be returned the lower portion of the riser by way of a downcomer. In this type of furnace, a smooth operation can be done such as heating of materials, reaction between materials, or drafting of the products by circulating the heat transporting material as described above.
However, the fluid bed furnace has a common problem to be solved in general in obtaining a smooth circulation of the heat transporting material as well as in controlling the quantity thereof. It is essential to obtain a smooth circulation of the heat transporting material for realization of the quantity control thereof. This realization of smooth circulation of solid particles mostly depends on the design of the downcomer. Namely, it is important to correctly select the height level of an end connection which opens at the lower part of a riser, and then it is important to determine the size of the downcomer in relation to the selected height level.
In the conventional technology, such determination of height level of the end connection and determination of the size of the downcomer were performed under totally different ideas from each other. That is, the former is determined from the total amount of the heat transporting material installed in the furnace, and the latter is determined from the calculated amount of heat transporting material circulating in the furnace. Whether the height level of an end connection is proper or not is judged from density of heat transporting material at a determined location when stirring the heat transporting material in the riser. However, since the density of heat transporting material at the location alters according to the parameters such as grain size or specific gravity of the heat transporting material, or velocity of the gas in the riser, it is impossible to evaluate the height level univocally according to the density of heat transporting material. That is, the pressure or quantity of pressure drop at the present location can be used for the justification of the height level of an end connection.
On the other hand, the judgment of the size of the downcomer is done according to the velocity of the heat transporting material in the downcomer as well as above mentioned calculated amount of the heat transporting material in circulation. Since these values also alter according to the parameters such as grain size or specific gravity of the heat transporting material, the optimization of the operation includes much difficulty.
In the conventional technology, in order to avoid such intricacy, a pooling device is provided at the middle of the downcomer for pooling the heat transporting material therein, to which a means for blowing compressed gas into the pooling device for sending the heat transporting material into the riser. By this method, it is necessary to put gas energy at an exalted state since the gas is blown into the furnace to raise the velocity of the heat transporting material which at first is zero or very small. It is also necessary by this method to distend the size of the downcomer since the total volume of flow increases due to the gas blow.
As described above, in the conventional method of making the fluid bed type furnace, there are some difficulties as follows. In determining the height of the connecting end from the total amount of the heat transporting material, or in determining the size of the downcomer from the calculated amount of heat transporting material in circulation, it is difficult to obtain correct values since these values cannot be determined univacally as described above. Thus, in the furnace designed after the conventional process, many problems will occur relating to the downcomer, such as blocking of the heat transporting material when the size of the downcomer is small, or decrease of the heat transporting material in circulation due to the generation of gas flow in a direction from the lower part of the riser to a cyclone separator by way of the downcomer (called as a "reverse gas flow" hereafter) when the size of the downcomer is large.
Otherwise, in the method of equipping a pooling device at the middle of the downcomer, not only is the size of the downcomer necessarily large due to increase of volume of flow occurring from the gas blow, but also a large energy is necessary for returning the heat transporting material by bringing them at high speed from stationary state or a state of very low speed.